The broad gain bandwidth of conventional fiber-laser systems allows for operation over a wide range of wavelengths, or even tunable operation. For the simplest fiber laser system with cavity mirrors having broad reflectivity, the output wavelength can be very broad and can vary with pump power, fiber length, and/or other parameters. In some cases, a fiber Bragg grating (FBG) can be used as a narrow-band reflector to limit the linewidth of the fiber laser system, but the range of linewidths that can be generated is limited, and methods and apparatus to extend the available range are not straightforward. For instance, the minimum bandwidth that can be generated with an FBG is typically on the order of 6-10 GHz for standard fiber and is even larger for polarization-maintaining (PM) fiber. An additional complication is that the fiber laser system using an FBG typically will operate only near the peak reflectivity, resulting in a laser linewidth that can be substantially less than the FBG bandwidth. Alternatively, nonlinear effects in the fiber can broaden the laser linewidth to be substantially greater than the FBG bandwidth, particularly for high-peak-power pulsed-fiber lasers, or even continuous-wave (CW) lasers that can often exhibit noisy, unstable output.
The power that can be generated from fiber lasers and fiber-laser amplifiers can often be limited by nonlinear optical effects in the gain and/or delivery fibers used in the system. In particular, Stimulated Brillouin Scattering (SBS) is a well-known phenomenon that can lead to power limitations or even the destruction of a high-power fiber-laser system due to sporadic or unstable feedback, self-lasing, pulse compression and/or signal amplification.
There is a need for laser systems, particularly fiber-laser amplifiers, where the linewidth of the emission to be generated must be engineered to lie within a certain range of values. This need can arise for instance, when a fiber-laser system must produce optical wavelengths that only lie within a narrow linewidth, e.g., for coherent detection, coherent phasing of multiple systems, or bandwidth acceptance of nonlinear optical processes. On the other hand, narrow linewidth can lead to some types of nonlinear optical effects in the gain or delivery fiber of the system, limiting the peak power that can be generated in such a system.
The optimum seed source for a fiber amplifier system would be stable, low-noise and produce a given linewidth as required for the particular application. If polarized output is required from the system, its polarization properties must be much better than the requirements for the output as well. The simplest, most robust, seed sources typically used are Fabry-Perot laser diodes or fiber lasers. These are multi-line and in the case of the laser diodes, the output can extend to several nm or more. FIG. 1 shows an output spectrum for a Fabry-Perot laser diode centered around 1060 nm. On this scale, the mode structure is apparent, with “gaps” in the spectrum. Multi-longitudinal mode fiber lasers typically have much narrower mode spacing and may operate on a much smaller number of modes, but the phenomenon is similar.
FIG. 1 shows an emission spectrum from a Fabry-Perot laser diode showing multi-longitudinal mode emission over a wavelength range of approximately 3 nm (about 900 GHz). One issue with this type of seed source is that the spectral distribution of power is not constant, but the power can fluctuate between the modes. This is known as “mode partition noise” and the timescale for redistribution of the spectral power can be on the same scale as the SBS build-up time of a few nanoseconds (e.g., about 5 to 10 ns). This can lead to the occurrence of SBS in the fiber amplifier system, even though the average linewidth of the seed source may be sufficient to avoid the SBS.
Single-frequency laser diodes or fiber lasers avoid the problem of mode partition noise and can be used for a number of applications. Particularly for situations where very narrow linewidth is required, these offer linewidths on the order of a few MHz (diodes) down to tens of kHz (fiber lasers). However, this linewidth is more narrow than actually needed for some applications and worsens nonlinear fiber effects such as SBS. For instance, the bandwidth acceptance for frequency doubling can be ten's of GHz so a linewidth narrower than a broad-band free-running fiber laser may be required but a single frequency source may be much narrower than actually needed and could lead to lower power or instabilities due to fiber nonlinearities.
What are needed are improved seed sources that help avoid the above-described problems and provide other benefits.